1. Field of the Invention
The present invention relates to a piezoelectric element driving circuit that drives a piezoelectric element.
2. Description of the Related Art
To persistently or moderately give a medicine to a living body, in general, intravenous drip injection (hereinafter, simply referred to as “drip”) is used. Medical equipment for carrying out the intravenous drip injection is referred to as “drip device”. In the drip device, one end of a tube is connected to a container that contains a liquid medicine, and the liquid medicine is injected into the living body through an injection needle attached to the other end of the tube. The tube is provided with a device for adjusting the injection amount of liquid medicine, which adjusts the injection rate (the injection amount per unit time) of the liquid medicine. As a device for adjusting the injection amount of liquid medicine, a device having a drip tube and a clamp has been used. With the conventional device, medical personnel, such as nurses, operate the clamp while viewing the dripping state of the liquid medicine in the drip tube.
Besides, a device, called liquid medicine injection pump, may be used to carry out the transfer and injection rate adjustment of the liquid medicine. In the liquid medicine injection pump, for example, an injection syringe is driven by a motor having a mechanism to control the number of rotations such that a liquid medicine is injected into a living body at a predetermined injection rate. When the liquid medicine injection pump is used, it is possible to automatically carry out the transfer and injection rate adjustment of the liquid medicine.
A portable drip device that is small and light-weight and driven with a battery has been developed. The liquid medicine injection pump using the motor as described above consumes high power, and is not suitably driven by a battery as a portable device and difficult to be downsized. Thus, a technologies has been developed in which a micropump that transfers a liquid medicine by an actuator using a piezoelectric element is applied to the liquid medicine injection pump for use in the drip device.
A micropump that uses an actuator constituted by a piezoelectric element has, for example, a structure in which the actuator is provided in one surface of a pressure chamber, through which a liquid medicine passes. In this micropump, a signal, such as square waves or sine waves, with a voltage varying in a predetermined cycle is applied to the piezoelectric element to vibrate the actuator. The vibration changes the volume of the pressure chamber through which the liquid medicine passes, and thereby the liquid medicine is transferred. The injection rate of the liquid medicine can be adjusted by controlling the frequency or amplitude of the signal applied to the actuator.
Japanese Patent Application Laid-open No. H8-109945 discloses a conventional technology in which an actuator is driven by using a sinusoidal signal. Japanese Patent Application Laid-open No. 2002-218772 discloses a conventional technology in which an actuator is driven by using a square-wave signal.
In an actuator for a micropump, piezoelectric elements, called unimorph or bimorph, are used. The driving voltage of such an actuator is relatively high, for example, around 100 V and it is difficult to control the frequency or amplitude by a small electronic circuit.
If square waves are used as the driving signal of an actuator constituted by a piezoelectric element, it is relatively easy to obtain a high-voltage driving signal by a chopper circuit or the like. However, a square-wave driving signal causes unnecessary vibration in an actuator, and noise at the time of driving increases.
An actuator constituted by a piezoelectric element is formed by bonding another piezoelectric element or a metal plate to a plate-shaped piezoelectric element. Thus, as the characteristic of the bonded structure, mechanical strength differs depending on whether the potential of the driving signal is positive or negative. For this reason, an actuator constituted by a piezoelectric element is not suitable for driving that applies sine waves having positive/negative symmetrical amplitude with no DC component for a long time.